Receive RF beam-forming is widely used as a mechanism to improve signal strength and/or reduce multi-user interference. On the other hand, in many scenarios, user scheduling for uplink transmission is not fully pre-determined. This is referred to as Random Access. WiFi systems are a notable example in this category. Other examples include “emerging M2M and IoT applications”. Hereafter, methods and systems described herein will be explained in the context of WiFi systems, such 802.11g and 802.11n.
In setups using Random Access, clients access the uplink channel without prior coordination. This means, the Access Point (AP) does not have any prior knowledge about the identity of the client which will next access the uplink channel. In this configuration, AP can determine the identity of the client sending in the uplink only after the preamble of the corresponding uplink packet is received and is successfully decoded. Hereafter, this feature is refereed to as Uplink Client Anonymity. This shortcoming makes it difficult for the clients who are in deep fade to even establish the link. For those clients that the uplink signal is strong enough to be heard by the AP (establish the link), being subject to deep fades will reduce the throughput and increases the delay. AP's are typically unable to adjust its antenna (beam-forming) pattern to provide each client with a better reception during this stage.
Due to Uplink Client Anonymity, Transparent Beam-forming in the uplink is more challenging as compared to the case of downlink. As a consequence, prior art in Transparent Beam-forming is limited to downlink transmission. In particular, some prior art relies on observing the packet-level error behavior in the downlink, and accordingly determines a transmit (downlink) antenna pattern for a particular client. The error behavior is gauged by examining multiple antenna patterns and selecting the one that minimizes the Frame Error Rate (FER), wherein FER is measured by counting the number of retransmissions (for any particular client in conjunction with the examined transmit patterns). As a result, methods based on the prior art are slow, and inefficient. Another disadvantage of such prior art techniques stems in their inherent reliance on observing erroneous packets to guide their beam-forming decisions. In other words, they can offer improvements only after several downlink packets are communicated in error. This shortcoming results in delay and reduces the throughput. Another shortcoming of prior techniques is that they are limited to downlink beam-forming, while beam-forming in the uplink is generally more important. The reason is that, in the downlink, an AP typically relies on better power amplifiers as compared to that of resource limited mobile clients. This inherent mismatch (downlink vs. uplink link quality) means that Transparent Beam-forming is actually more important for use in the uplink, yet solutions do not exist.
One reason that Transparent Receive Beam-forming has not been used in network setups using Random Access is that beam-forming coefficients for each client should be selected based on the particular channel realization corresponding to that client, and the AP does not know which client will next occupy the uplink channel.
A challenge in Uplink Transparent Beam-forming concerns computation and tracking of the proper beam-forming weights for each client. These features limit the abilities of the AP in adjusting its receive beam-forming weights in a timely manner in order to provide the best receive gain for the particular client that is occupying the uplink. On the other hand, in transmit beam-forming, the access point is aware of the identity of the client that will be next serviced in the downlink (prior to starting the transmission), and accordingly, can adjust its beam pattern according to the particular client.